In cryptography, a secure communication channel can generally be provided if two legitimate users have access to a common, secret key. One of the difficulties in secure communications is to make sure that each legitimate user obtains the secret key without interference or eavesdropping by a non-legitimate party. As such, many secret key distribution systems have been developed over the years. Most recently, cryptographers have begun using quantum techniques to securely distribute/create secret keys (called “quantum key distribution” (QKD)). For example, QKD protocols include the 6-state protocol or the BB84 protocol.
QKD protocols generally comprise two phases: a quantum phase and a classical phase. In the quantum phase, quantum states (for example, qubits) are distributed via a quantum channel. The nature of the quantum channel is such that it is possible to detect if the signal has been tampered with and this allows for stronger security in the secret keys. Upon receipt of the quantum signal, the legitimate users measure the quantum states to obtain classical information about the quantum states. In the classical phase, classical information is exchanged using a specified communication protocol over the classical channel to determine the secret key.
One of the difficulties of quantum key distribution protocols is that quantum channels cannot be established in certain circumstances. For example, quantum states are typically communicated using photons, for example, via optical fiber or through free space (line of sight). In either case, transmitting is limited by signal loss over distance and, particularly in the free space case, may also be limited by environmental factors. In optical fibers, the transmittance of the quantum signal is generally limited by loss which grows exponentially as the distance increases based on, for example, the loss coefficients of the optical fiber. It is anticipated that maximum distances will be a few hundred kilometers.
In order to overcome this problem, conventional systems make use of intermediate nodes between the two legitimate users. These intermediate nodes may be part of a trusted repeater network or a proposed quantum repeater network.
In a trusted repeater network, one or more trusted intermediate nodes are provided between the legitimate end users and point-to-point communications are used among the legitimate users and the trusted nodes. In the point-to-point communications, the QKD protocol is used to establish a secret key between the first legitimate user and the trusted node and then, in the case of only one node, between the trusted node and the second legitimate user. If there are multiple nodes, a secret key would be established between each intermediate node as well.
In a proposed quantum repeater network, the legitimate users each create a maximal entangled state and each keeps one entangled state subsystem and sends the other entangled state subsystem to an intermediate node over a quantum channel. The intermediate node saves these quantum states into quantum memory and performs a Joint Bell measurement on the arriving signals and announces a Bell measurement result via the classical channel. This produces a quantum correlation that is shared between the legitimate users (that is, an entangled state). The entangled state is then used to complete the QKD protocol and establish a secret key, without further involvement from the intermediate node.
While trusted repeater network systems can be effective, it is often necessary to have a large number of complex intermediate nodes between legitimate users or among legitimate users in a network. This can lead to additional costs and complexity. Further, quantum repeater networks have yet to be practically implemented. As such there is a need for improved systems and methods of quantum key distribution.